An image sensor is a device capturing and converting an impinging electromagnetic radiation such as a light flux into an electronic signal. In digital imaging, Active-Pixel Sensors (APS) are mostly used. APS are image sensors consisting of an integrated circuit containing an array of pixel sensors, and wherein each pixel contains a photodiode and an active amplifier.
In an APS, the photodiode is sensitive to incident light. More precisely, the photodiode converts the incident light into charges which are accumulated during a given exposure time and then converted into an amplified voltage inside the pixel. This voltage is a continuous analog physical quantity which can be converted, thanks to an analog-to-digital converter, to a digital number representing the voltage amplitude.
One of the main problems of standard pixels is their potential saturation appearing when too strong incident light and/or too long exposure occur. In range imaging system using Time of Flight technologies (ToF), for example a Time-Of-Flight camera system 3, as illustrated in FIG. 1, providing distance information by analysing the Time of Flight and the phase of a pulsed light signal emitted 16 by a controlled light source 18 and reflected back 17 by objects from the scene 15, the saturation may occur when objects having standard reflective properties are closer from the distance range the imaging system 3 is calibrated for. The object reflects at that time too much from the emitted light and causes at least some pixels of the sensor to respond at their maximum value. The saturation may also occur when objects demonstrates specular reflective properties in the wavelength domain the pixels have been designed to be sensitive to, such as when a mirror in a scene reflects the entire incident light it receives onto the sensor imaging the scene, or when objects reflect and concentrate the incident light onto a portion of the sensor, or when an external light source emitting a strong illumination in the same wavelength domain the ToF camera has been designed for is illuminating the sensor.
When pixels are saturated, meaningful information about the scene is lost since the response provided is flattened at the maximum voltage value that can be provided; this leads to image artefacts or defects such as burned area, blooming effects in images. Moreover, certain applications, for instance the computation of depth information in ToF technology, uses phase shift based computations from a plurality of captures to derive a distance measurement. If pixel saturation occurs during integration time, the voltage at the detector nodes reaches a saturation level which corrupts the corresponding capture.
Another main problem of standard pixels is the fact that noise can be very strong. If the signal/noise ratio is small, then the noise is preponderant during the capture and useful information is lost.
An important figure of merit of an imaging sensor, taking into account both saturation and noise parameters, is the so-called Dynamic Range (DR), illustrated in FIG. 2. The Dynamic Range can be defined by the following ratio in decibels:
  DR  =      20    ⁢                  ⁢          log      10        ⁢                  signal        ⁢                                  ⁢        maximum                    noise        ⁢                                  ⁢        floor            
For the purpose of increasing the Dynamic Range of image sensors, several techniques have been implemented. A first solution for increasing the Dynamic Range of image sensor has been to reduce the level of the noise floor, for instance by reducing the size of the sensors. This strategy suffers from the drawback of decreasing at the same time the saturation level of the sensor. This is the case A illustrated in FIG. 2.
Another approach for increasing the Dynamic Range of sensors is to increase the saturation level of the sensors. Several solutions of High Dynamic Range (HDR) or Wide Dynamic range (WDR) systems have been proposed in standard image sensors using several electronic circuits with addition of latches and/or memory point. Sensors have also been designed with techniques such as well adjusting, multiple captures or spatially varying exposure. Moreover, extra logic circuitry has been added per CMOS APS, but this reduces the effective sensitive area of sensor and results in a very low fill factor that do not comply with efficient ToF imaging requirements.
Another solution consists in using circuits with logarithmic pixels. Such pixel circuits generate a voltage level that is a logarithmic function of the amount of light striking a pixel. This is different from most CMOS or CCD type image sensors that use a linear type of pixels. Nevertheless, the use of logarithmic pixels complicates highly the post processing to compute required data, as depth information for instance, since it introduces well known compression issues and request also extra processing computations.
One of these solutions, based on the increase of the saturation level, is illustrated by FIG. 3. An extra capacitor CPA is used, on which the charges generated during the integration time in the photodiode PD can be transferred. The main drawback of this method is that, once transferred on the extra capacitor, only one read-out cycle is possible. It is not possible to read-out several times the data contained on the extra capacitor, and to adapt the conversion gain to be used.
A solution remains to be proposed for increasing the Dynamic Range of Time-Of-Flight sensors, while allowing non-destructive multiple read-outs of the same charge information using different conversion gains.